a) Field of the Invention
The present invention relates generally to apparatus and method for preventing overload applied to any one of a plurality of semiconductor switching devices installed on arms of an inverter main circuit (hereinafter, also simply called an inverter circuit or a motor drive) for a motor-driven electric car (also called an electric vehicle).
The motor is driven by means of an AC power, the AC power having a plurality of phases and into which a DC power is inverted by means of the plurality of semiconductor switching devices, and the electric car is caused to run through the motor.
The present invention, more particularly, relates to the apparatus and method described above in which, when the electric car is trying to run up-hill (running on an ascending slope) with the motor locked, a state in which an excessive current flow causing an overload through any one of the switching devices which is presently driving the motor is prevented from occurring and which can easily be escaped from the motor locked state.
b) Description of the Related Art
Such a type of the electric car as described above often involves such a case as not being started to run when an accelerator pedal is slowly depressed during the run on the ascending slope. That is to say, the electric car has the motor developing a predetermined output torque according to a depression depth of the accelerator pedal and is started to run when the motor develops the output torque which is larger than a running resistance against the vehicle. If the driver depresses slowly the accelerator pedal, a variation rate of the output torque of the motor is so small that it takes a lot of time for the vehicle to be started to run when the running resistance against the vehicle is relatively large as is the case of the vehicular run of the ascending slope. The motor is locked for a time duration until the electric car is started to run. When the motor is locked, a particular phase current is continuously caused to flow through only the particular one of the switching devices of the inverter circuit which supplies the particular phase current to the motor.
When the locked state of the motor is continued for a long period of time, a temperature of the current flowing through the particular one of the switching devices in the inverter circuit is raised. A control of limiting the output of the motor due to a temperature protection of the current flowing through any particular switching device is performed.